It is known that a polymer having an alkenyl group at a molecular chain terminus undergoes crosslinking either by itself or in the presence of a curing agent such as a hydrosilyl-containing compound to give a heat-resistant, durable cured product. The main chain structure of such a polymer, so far known, includes polyether series polymers such as polyethylene oxide, polypropylene oxide, polytetramethylene oxide, etc.; hydrocarbon series polymers such as polybutadiene, polyisoprene, polychloroprene, polyisobutylene, etc., inclusive of hydrogenation products thereof; polyester series polymers such as polyethylene terephthalate, polybutylene terephthalate, polycaprolactone, etc.; and polysiloxane series polymers such as polydimethylsiloxane, among others, and these polymers have been used in various applications selectively according to the characteristics of the respective main chain structures.
Vinyl polymers have several characteristics not shared by the above various polymers, such as high weather-resistance, heat resistance, oil resistance and transparency, among others, and the use of the polymers having side-chain alkenyl groups has been advocated in the field of weather-resistant coatings (e.g. Japanese Kokai Publication Hei-3-277645; Japanese Kokai Publication Hei-7-0399).
However, alkenyl-terminated vinyl polymers cannot be easily produced and, therefore, have not been much employed on the industrial scene as yet.
Japanese Kokai Publication Hei-1-247403 discloses a process for producing an acrylic polymer having an alkenyl group at both termini which comprises using either an alkenyl group-containing dithiocarbamate or diallyl disulfide as the chain transfer agent.
Further, Japanese Kokai Publication Hei-6-211922 discloses a process for producing an alkenyl group-terminated acrylic polymer which comprises preparing a hydroxyl group-terminated acrylic polymer using a hydroxyl group-containing polysulfide or an alcoholic compound as the chain transfer agent in the first place and taking advantage of the reactivity of the hydroxyl group so introduced.
Meanwhile, curable compositions adapted to give rubber-like elastic products on curing have been used broadly in the field of adhesive, sealing and shock-absorbing materials. These compositions can be roughly classified by the mode of curing into the so-called moisture-curable compositions which are stable in a sealed environment but cure in the atmosphere by absorbing its moisture even at room temperature to give rubber-like elastic products and those compositions which are adapted to crosslink through hydrosilylation or the like reaction on exposure to heat.
However, by the above prior art technology, it is difficult to introduce an alkenyl group into the polymer terminus with good reproducibility. Moreover, since the standard radical polymerization reaction is utilized in these processes, there is the problem that the molecular weight distribution (the ratio of weight average molecular weight to number average molecular weight) of the product polymer is as broad as 2 or more and, therefore, the viscosity of the polymer is high. In utilizing the polymer as a sealant or an adhesive, for instance, such a high viscosity makes it difficult to handle the polymer and, moreover, a filler for reinforcement cannot be formulated in a sufficient amount.
It has also been found difficult to introduce a (meth)acryloyl group, which has radical polymerization activity, into a vinyl polymer which is synthesized by radical polymerization. Particularly, few compounds with a (meth) acryloyl group introduced into the oligomer terminus have so far been successfully synthesized.
In many of photocurable compositions so far available, a low molecular compound having a (meth)acryloyl group is employed. In such cases, the odor caused by vaporization of the unreacted low-boiling compound during and after curing has been a serious problem. To avoid this trouble, an oligomer having a (meth)acryloyl group is employed. However, chiefly from synthetic points of view, such oligomers are limited to epoxy-acrylate, urethane-acrylate, polyester-acrylate and the like systems. Moreover, oligomers having large molecular masses are not available. As a result, such compositions tend to give comparatively hard cured products and do not give cured products having good rubber-like elasticity.
In view of the above state of the art, the present invention has for its object to provide a vinyl polymer having a reactive functional group at a molecular chain terminus at a high rate and a curable composition comprising the polymer.